Heat treating furnace and method of heat treating



N. DAVIS 2,075,929 HEAT TREATING FURNACE AND METHOD OF HEAT TREATING Aprl 6, 1937.

Original Filed Aug. 14, 1935 v 5 SheefLs-Sheet 2 NI/ENTOR LEW/JNEWELL DA /s L. N. DAVIS 2,075,929

HEAT TREATING FUBNACE AND METHOD OF HEAT I `RE`ATING April 6, 1937.

Original Filed Aug, 14, 1935 5 Sheets-Sheet 3 NVENTOR L 5 w/s NE wa L DA ws Fw IL ATTORA/EX April 6 1937. L. N. DAvs 4 1 3 HEAT TREATING FURNACE AND METHOD OF HEAT TREATING Original Filed Aug. l4, 1935 5 Sheets-Sheet 4 [NL [NTOR LE was` N: wa L DA ws MWW;

April 6, 1937. L. N. DAVIS 2,075,929

HEAT TREATING FURNAE AND METHOD OF HEAT TREATING Original Filed Aug. 14, 1935 5 Sheets-Sheet 5 A m w fNI EN 7'OR ATTORNEX w f a L: W/.S/VEWELL DAV/S Patented Apr. 6, 1937 a UNITED STATESPATENT OFFICE HEAT TREATING FURNACE AND METEO!) OF HEAT TREATING Lewis Newell Davis, Santa Monica, Calif., as-

signor to Douglas Aircraft Company, Inc., Santa Monica, Calif., a corporation of Delaware Continuation of application Serial No. 36,126, August 14, 1935. This application February 3, 1936, Serial No. 62,037

20 Claims. (Cl. 263-52) My invention relates to the art of heat treata urnace in which the heat-treating chamber is ing, and relates particularly to a unique heat heated principally by convection, and in which treating urnace and also to a unique method of small amounts of radiated heat are utilized forrasing articles or objects to the desired temperaassisting in maintaining substantially equal temture and maintaining the articles at such temperatures in difierent parts of the heat-treating 5 peratures over a period of time without such chamber of the furnace. temperatures varying more than a few degrees. A further object of'my invention is to provide a The present application is a continuation of my furnace in which the heat-treating chamber is application for Heat treating furnace, Serial No. principally heated by circulating preheated gas 36,126, filed August 14, 1935. therethrough, and in which the temperature in '10 In certain types of heat treatment it is neces the heat-treating chamber is controlled by consary that all portions of the object being heat trolling the temperature of the circulated gas treated be' brought to substantially the same temand by radiated heat applied to different parts perature or condition of heat. In many instances of the heat-treating chamber.

this is done by immersing the object in a bath It is a still further object of my invention to 15 which is maintained at the desired temperature, provide a furnace of the character pointed out and also heat treating Operations of this general in which the temperature within the furnace is character have been carried on with more or less additionally controlled or moderated by the insuccess in heated air or gas-filled chambers. troduction of a cooling gas into the heat-treating Both of the foregoing methods have disadvanchamber at certain parts or sections thereof as tages which I believe are practically overcome required by the temperatures existing therein. in my present invention. For example, the sub- In the furnace constituting the preferred form stances used for heat treating baths enter small of my invention and which is to be described openings and grooves, such, for example, as hereinater, the heat-treating chamber is divided seams in fabricated metal articles, and are difliinto sections and a separate heating means is '25 cult to remove therefrom, and in heat treatment applied for each section. It has been found in by use of air-filled heated chambers a sufliciently practice that when the heating means for a givenaccurate control of the temperature is not always section is de-energized or rendered non-operative, obtainable, and due to unequal distribution of there is a tendency for the section to continue 3 heat within the chamber all parts of the object to rise in temperature due probably to the fact being heat treated, or objects disposed in difierthat the heating means for the section continues ent zones of the heat treating chamber, are subto give ofi heat.

jected to different temp'eratures. It is accordingly an object of my invention to It is an object of the present invention to proprovide a furnace in which there is means which vide a heat treating furnace in which a very compensates for the heat supplied by the heating accurate control of temperature is obtained, in means after it has been de-energized to prevent which the distribution of heat is very uniform. an upward coasting of temperature.

In the following part of the specification the It is a further object of the invention to provide term "air" is used in reference to the heat cona heat treating urnace in which the heat produciu veying medium. It should be understood that ing elements are arranged in sections within the 40 this term comprehends the use of various suitfurnace in such a manner that each sectionmay able gases. have the heating thereof individually and sepa- It is also an object of my invention to provide rately controlled; wherein the heat producing a heat treating furnace wherein the air is treatmeans are of electrically energized type employ- .r ed in a heat receiving zone within the furnace ing heating elements having associated switch and moves as the result of convection to the heat mechanisms for the control of the same in a. transmitting zone of the urnace in which the manner to maintain the temperature in the vaarticles to be heat treated are placed. rious sections of the heat treating furnace with- `A further object of the invention is to provide in very close limits; wherein thermocouples are a device of the above character having curtains employed in the sections of the furnace as heat 50 of heat insulating material for substantially preresponsive means for indicating the temperatures venting the 'passage of heat from the heat proexisting therein, together with a control unit ducing elements of the furnace by radiation to with automatic means for progressively connectthe articles to be heat treated. ing the same to the successive thermocouples and A further object of my invention is to provide the control switches for the electric heating ele- 55 which will not va 'ments which are associaterl therewith in the separate sections of the heat treating furnace.

It is a still further object oi my invention to provide a heat treating fumace of the char cte:: pointed out in which the temperature throughout the entire space within which the articles are subjected to heat treatment is maintained very accurately at a precietermined point, and in which hot spots a'e preventecl by circuating the air from one portion of this space to o'the:: portions the'eof.

Et is another object my invention to provide a heat treating furnace of the characte? pointed out to provide whi prevent ao upward coasting of tempe ature V ieri the nace nas been heatecl to maximum temperature and the hee* x has beenshut off.

The fu'nace inco'poratiog the unique features advantages outlinecl in the prececling cbjects and statements of intention is particularly designed to perfo me a novel which I have inventecl. me iod is very satsiacto'ily and efciently ;ero med in the rnace of my inventien, ye* i may 'oe' piacticei in slightly diei'ent types whicl`- suitahiy designed GY eqto' eei perform g my method. E desire, there ore, to detain patent protection not only on th on the methoci which I li Et is accordingly ali to provide a method of sea" e Whic Jill heat the objects to t It is anothei` Vide a i ethe& e principally by neate-a aii* ula tact with the objects, i ably ositicned what shembet in orcle' be maintained.

Zt is a stili further oc o inveiiti o provide a method in which objects are he* by leated as circulatecl into t e'cperatu'es are co:-

- a the heat treat g and radiation, and is mcderatecl hy suit ing gas.

objects and as" *ic-u will be made evident i g na of the specific Reerrng to the -ri'awings witch 'ig. l is a cross-eesti@ view taken substaiitially as indicated by the line 3 S of 4 is a complete wiring intention.

5 is a Wiring diagram tiou of the electrical control.,

heat' aomeae nectecl with the main control unit of the in-' vehtion.

Fig. 6 is a cross-sectiohal View through an altenative form of furnace of my invention ancl which illust'ates a design pai'ticularly adapted for smaller sizes of furnaces.

The form of the inventioi discloseci in the drawings employs a chamher ili. Extending between side walls ll and. i? are top and 'cottom walls iii and it, a Isai' enti wall and. front moi/able wall i@ forming a door for the opening li of the chamhe` These walis which form the chamber are of suitable beat insulating material, and the entire structuse may be supported by the use of structural mem" S bei-s and ti).

Placed within the walls which form the chant. bei* are curtain walls 01' insulat'oo walls to. These walls are pcsitioned quite close to the side Walls ii and t and clivicle the chambe: ili into a heat-treating chamber and two eating chambei's The curtain wa s stop` short of the lip-per and lower ?sounds o the chamber so as to provide uppe' open ihgs or passages lower cpeni sages connecting the ch ioe's e'aliy indicated as E and specifically io cat as El, ES, Et, and ere of each of these elements :o'mo 3:5, i pair of elements constituting a section whz is incliviclually or sepaiately controllecl and oc copies its individual section within the gas-hoat chambers and the heating chamhe' H As shown in 2, one. each of the eleme is placed on hand sicle wall i l of the chamoe' i, and

elements and are placed. on the right-hand wall 52 of chanhe* The general arrangem nt of elements E is show? in l am" whe'eic the pair of heat' elemento the pair of heating elements are sho oppc 'ng relation on the sicle Walls of the heating chamber Ou a suitable trac mea.-

ja the bo 'i e r carriages are supported ?Jy Tb s whereby th y ay be i'ollecl of heating sham' Lil. The

ca loading mola-s which occupy v sp ce the chamher between the .ame

walis thereof. These loading racks aire suz's 55 port l in elevatecl relation solely hy the wheels so that conc'uction of at ;from the the articles to he heat tteateci is su' eliminatecl owing to the that t.. .e loading racls receive substaotiaiiy tioii (lus to contact with the Welle ozurnace and method of my een particularly createcl for t. Qent of relativeiy thin .metal alio example as cluralunn, whi e* ie Construction of oiope'l hoat treat this alumimu'z quite necessary that the temperature different parte of the heat-ti'eat' .held within close temperature limits that all ;oortions of given objects placed in different in order object or different parts of the heat t'i-catia? chamber 'oe heat treatecl eizo. to me emaili J shown with its inlet end 32 i ato-76,929 r V 3 desiredresult has been accomplished by heating the heat treating furnace principally by convection and by supplying necessary additional heat by radiation. As shown in Fig. 1, the heating elements El to ES inclusive are placed in the gas-heating chambers MB and when these heating elements have been energized heat is applied to the air or gas within the chambers !OB and it is caused to flow upward, and when this occurs cooler air will move in an upward direction and be heated. In this way a continuous closed circuit circulation, as indicated by the arrows 29, is' set up. In the larger size of furnace as illustrated in Figs. 1 to 5 inclusive there are two separate circuits through which the hot gas flows,

one from each side of the heat treating chamber V IDA as illustrated. It will be seen that the heated gas flows upwardly through the gas-heating chamber IOB, inwardly through the passages 26, downwardly through the heat-treating chamber IDA, outwardly through the passage's 21. and into the lower ends of the gas-heating chammrs IOB, thus completing the circulation of the hot gas through a closed circuit. whenever a given heating element is energized heat is applied to the adjacent portion of the adjacent curtain wall 25, and although the curtain wall is made from a heat insulation material, such for example as Firecrete. a small amount of heat is transmitted through the curtain wall and into the adjacent part of the heat-treating chamber IuA. This action occurs adjacent each of the twelve heating elements whenever any of the heating elements is energized. It will therefore be seen that in addition to the temperature derived from the hot gas circulated through the heat-treating chamber iuA there is at certain `points and at certain times additional heat applied due to radiation, as will be pointed out hereinafter when a complete description of the operation of the iurnace is given. This radiated heat is quite valuable in assisting in maintaining an even temperature throughout the entire heat-treating chamber IDA.

To produce an even distribution o! the heat within the chamber the inventionprovides a means u for taking air from one part of the chamber and returning it to other paris of the chamber. With reference to Fig. 2, a duct 3| is connected to that part of the chamber deemed to have the lowest temperature during the normal operation of the heat treating furnace. This specified portion of the chamber is the lower part of the forward end of the chamber adjacent the door IS. The duct 3| is connected to a blower or fan 33 having an impeller 34 mounted on a shaft 35 carrying a pulley 36 adapted to be driven by a belt from a motor 31. So that a high temperature will not be carried to the belt, which is not shown. radiation flns 38 are mounted on the 'outerportion of the shaft 35. The blower 33 discharges into a vertical pipe 33 which extends upwardly within the rear wall |5 to a laterally extending header 4i. As shown in Figs. 2 and 3, six delivery pipes PI, ?2, P3,' P4, P5, and PS extend forwardly in -the upper wall |3 of the chamber o, these air distribution pipes, generally designated by P, having the function of distributing air which has been taken from the chamber !0 through the inlet end 32 of the duct 31 to the six sections of the chamber o containing or established by the pairs'of heat producers or heating elements includes branch pipes 42 which drop on opposite' sides of the chamber |0- and across the inner faces of the curtain walls 25 to a. plane substantially intermediate the height of the` chamber o so that the outlet ends 43 of the branch pipes 42 will delive' air downwardly and laterally into the three lower sections of the chamber o lying between each pair of heating elements E I, E3, and ES; The air circulation pipes P2, P4, and PS have branch pipes 44 forming the ends thereof, these branch pipes extending downwardly at the sides of the chamber o and a short distance across the upper portions of the curtain walls 25, so that the outlets 45 thereof will discharge air into the three upper sections in the chamber o lying between each pair of upper heating elements E2,`E4, and ES.

The withdrawing of air from one part of the chamber u and returning the same to other parts of the chamber n produces an air circulatio'n within the heating chamber Supplementary to the circulation of heated air i-ndicated by the arrows 29 oi Fig. 1. As shown in Fig. 2, there is also a longitudinal circulation toward the leftward or front end of the heating chamber and an intermixing efiect as indicated by the arrows 46. The circulation of air above described is for the purpose of producing an even distribution of heat through the central portion of the'heat-treating chamber by the elimination of dead spots in the circulation, and this circulation of air has a further important purpose in minimizing the upward coasting of the temperature in the heat-heating chamber ioA.

Each of the heating elements E includes a resistor 41 wound to spiral form and from each end of each resistor 41 a conductor bar 'vertically extends to the exterior of the chamber o, these conductor bars being included in the electrical circuits shown in the electrical diagrams, Figs. 4 and 5.

Associated with each section of the chamber, and likewise with each section of the heating elements, is a thermocouple T. Since there are six of these thermocouples, they are separately indicated as TI, T2, T3, T4, T5, and TS. These thermocouples are of the standard commercial type and include 'tubular casings which project through a wall of the chamber o, as shown in Figs. l and 3. These thermocouples accordingly constitute heat-responsive members which generate the electrical energy in proportion to the temperature te which they are subjected, and accordingly each thermocouple constitutes a means for checking the temperature in a separate section of the chamber o.

In the wiring diagram, Fig. 5, which is supplementary to the wring diagram, Fig. 4, I show u the manner in which a thermocouple Te is employed in conjunction with other control mechanisrns for the accurate control of the temperature in the heat-treating chamber lllA. From cne side of the thermocouple TB, a conductor 5! is carriecl to a stationary contact CE of a distributor 52 having five other stationary contacts CI, C2. C3, C4, and C5 arranged in a circle with the contact CG. The other side o f the thermocouple TS is connected through a wire 53 with a buss wire 54 extending to a main control unit which is of an electrically responsive and operative type. the distributor 52 is connected through a conductor 51 with a terminal 53 which is complementary to the terminal 59 to *which the. buss A rotary brush 56 forming a part of E. Each of the circulation pipes Pi, P3, and P5 wire 54 connects. As described, the thermocou- 5 strument which cooperates with the thermocou pie T6 in forming what is known as a potentiometer pyrometer having a galvanometer which operates in response to the electrical energy produced by the thermocouple T6 to control electri- 10 cal mechanisms which need not be shown, such electrical mechanisms moving a switch contact -6l to the left or to the right in accordance with the rise and fall of the temperature to which the thermocouple T6 is subjected.

When the movable contact 6| engages the low temperature contact L, it connects a conductor 62 through a wire 63, the contact Bl, the contact L, a conductor 64, a rotary switch brush 65 of a distributor 65a, a contact AG, and a conductor 66 with an electromagnet 61 which is in turn connected to conductors 68 and 69 with a main conductor 18 which cooperates with the conductor 62 and an additional main conductor 'll to form` a three-wire three-phase electrical distribution system which may be connected to the three wires 12 of Fig. 4, which are brought into the building through a main switch 13, from a commercial electrical supply system or from any suitable source of three-phase electrical current. As will be noted in Fig. 5, a switch 14 is interposed between the conductors 68 and 69, this switch being normally closed unless opened as a result of the energizing of an electromagnet 15, as will be hereinafter explained.

As shown in Fig. 5, the energization of the electromagnet 61 results in the closing of a. hold in relay 16 and a controlrelay TI. The closing of the control relay 71 closes a circuit consisting of a conductor 78, an electromagnet 19, a

conductor 8I, and a conductor 82, with the re- 'sult that a control switch 83 is closed by the electromagnet 19; thereby closing a circuit 84 which connects the heating elements E6 with two wires 10 and 'H of the three-phase supply system, and resulting in the generation of heat within a section of the heat-treating chamber air control switch 86 which engages a. contact 81 when the electromagnet 85 is de-energized, and is drawn into engagement with a contact 88 when the electro-` 'I'he switch 86 is connected by a wire 94 with the conductor 18. The solenoid coils 89 and 9| are adapted to move a core 60 or armature 95 which is connected to a butterposed in the forward ends of the air distribut- 65 ing pipes PI to P6 for control of the circulation of air therein. As shown in' Fig. 2, each valve V has a supporting stern or shait 96 with a handle 91 formed thereon, each handle 51 being connected to a solenoid 95 as indicated in Fig. 5. TO When the switches 11, 83, and 86 are' in the positions in which they are shown in Fig. 5, the solenoid windng QI will be energized and the valve Vs will be in a closedor flow-restricting position, as shown in Fig. 3. When, however, the 75 movable switch s of the main control unit theparts are shown in neutral moves into engagement with the contact H and the electro-magnet 85 is d a-energized as a result thereof, the switch 86 will be moved into engagement with the contact 81 and the solenoid winding 89 will be energized, moving the armature 5 95 so as to rotate the valve V6 into an open position. Accordingly, during the time the heating elements E6 are energized and are giving off heat so as to raise the temperature in the corresponding section of the chamber I 0. the flow through 10 the outlets 45 of the air circulation pipes Ps will be of reduced quantity or completely shut ofl as desired, depending upon whether or not the valves are arranged to completely or only partially close the pipes when in a restricting position. 15

When the deiivery of heat into the chamber IO produces a sufliciently high temperature in the thermocouple to cause the movable switch member 6! of the main control unit 55 to move into engagement with the' contact'H, as indicated by 20 dotted lines !00 in Fig. 5, current will flow through a conductor loi to a rotary switch member or brush i 02 forming a part of a distributor !03 having a plurality of contacts generally indicated as B and specifically indicated as BI to B6 25 inclusive. The rotary switch element l 02 engages the stationary contact BB and is connected through a conductor IM with the electromagnet 15 which is in turn connected through a wire |85 with the conductor 69. The energization of 30 the electromagnet 15 opens the switch 14 and accordingly interrupts the flow of current through the electromagnet 61, allowing the switches' 16 and TI to open.

The opening of the switch TI de-energizes the 30 electromagnet 19 resulting in the opening of the un switch 83 which in turn de-energizes the heating elements Et and the electromagnet 85. De-energization of the electromagnet 85 permits the switch 86 to return to engagement with the con- 40 tact 81 so that the solenoid winding 89 will be energized, moving the armature 95 so as to swing the valve V6 into fully open position such as shown in Fig. 4, thereby permitting a maximum flow of air to be circulated through the air circu- 45 lation pipe 1 6.

As herein mentioned, the use of the air circulation has a function of reducing "coasting" of the temperature. Prior to the opening of the switch 83 the heating elements E6 are heated to a maxi- 0 mum temperature, and being of metal have heat stored therein which continues to pass into the chamber o after the opening of the switch 83 and accordingly tends to raise the temperature above the value at which the control means is set 5 I to open the switch 83. In the invention a circulation of air is increased at the time the switch 83 is open. This increased delivery of air into the section of the chamber identified by the heating elements ES, which have just been shut ofl, has the purpose of ofisetting the tendency oi the temperature to increase greatly above the prescribed value due to the giving oif of stored heat from the elements E6, the chamber walls, and the curtain walls 25, after the switch 83 has been 85 opened.

A motor o is connected through a circuit I I with the conductors 62 and 10. This motor is connected to the distributors 52, !03, and a, so as to rotate the brushes or rotary contacts 56, oz, and 65 thereof in synchronism so that they will successively engage their respective contacts CI to C6 inclusive, Bl to B6 inclusive, and Al to AS inclusive. As shown in Fig. 4, in which positions. each through the wire 66 and thermocouple T is connected to the buss wire 54 extending to the contact 59 of the main control unit 55 and each thermocouple T is likewise connected to its corresponding contact C of the distributor 52 by means of additional conductors 5!. Each of the pairs of heating elements E has a duplicate of control means shown in Fig. 5; that is, each pair of heating elements EI, E2, ES, E4, ES, and E6 has a circuit opening switch 14 equipped with an electromagnet 15 and relay switches 16 and 11 actuated by an electromagnet 61, together with the various switches 83 and 63 previously described in Fig. 5. Each circuit opening electromagnet 15 associated with each of the pairs of heating elements E! to ES inclusive is connected through a wire !04 with a corresponding contact B! to BG inclusive of the distributor !03. Also the relay electromagnet 61 associated with each of the pairs of heating elements E! to ES is connected through a wire 66 with a corresponding contact A! to AG of the distributor 65a. Accordingly, as the contacts 56, !02 and 65 rotate counterclockwise in synchrony, the main control unit will be progressively connected to each successive section of the electrical equipment, and the thermocouple T of said section will deliver electrical energy to the main control unit 55 to cause a proper movement of the switch part 6! to the right or to the left, with the result that the heating element E of the pair connected to the main control unit 55 will be turned on or off as required by the existing temperature in the section, and the valve controlling switch 86 will be likewise actuated as described relative to Fig. 5.

The hold-in switch 16 connects a conductor !!5 with a conductor !!6, as shown in Fig. 5, so that after the switches 16 and 11 are closed, further energization of the electromagnet 61 its associated contact A will not be required. Therefore, the heating equipment of each section may continue in "turned-on" position after the distributors have connected the main control unit 55 to a succeeding section or sections. The distributors 56, !02 and 65a are preferably driven at a speed of two revolutions per minute with the result that each of six sections of the heating equipment of the heat treating furnace is connected to the main control unit twice during each minute, or at intervals of onehalf minute. This assures a complete and practical automatic supervision of the sections of the furnace, with the final result that the temperature in all sections of the furnace is kept within a very small range of difference relative to a prescribed temperature 'for which the main control unit 55 is set. With the control equipment hereinbefore described, it has been found that the temperatures within the chamber are maintained to approximately five degrees, plus or minus, of the temperature setting of the main control or pyrometer unit 55.

In Fig. 4 a transformer !20 is shown having its primary !2! connected to the main circuit conductors 62 and 10, and having its secondary !22 connected with the main control unit 55 to operate potentiometer devices of this character.

As a safety measure, a safety circuit !23 is provided, this safety circuit including a fuse !24, an electromagnet !25 which holds the main switch 13 closed, and a switch !26 associated with the main control unit 55, which switch !26 is automatically opened in the event of failure of a thermocouple circuit. Should the temperature the power means ordinarily employed in within the furnace rise to a value above the melting point of the fuse !24, or should a thermocouple circuit fail, the safety circuit !23 will be opened, de-energizing the magnet !25 and causing the opening of the switch 13,thereby shutting down the entire system. Also in Fig. 4 the motor 31 or the blower 33 is shown connected to the conductors 12 through a switch !28.

In the performance of the method of my invention and in the operation of the heat treating furnace which I have described, when it is desired to utilize the furnace the doors are closed and the apparatus set into operation in order to bring the heat-treating chamber !BA up to the desired temperature. various apparatus illustrated in Figs. 4 and 5 is suitably adjusted so that the parts will operate as previously described to maintain the temperature as desired. When the furnace is cold all of the heating elements E! to ES will be energized With the result that the circulation of heated gas will be set up as indicated by the arrows 29 in Fig. 1, and in addition heat will be radiated through the curtain walls 25 at points adjacent the difierent elements E! to ES inclusive, as previously pointed out. Most of the heat' for heating the heattreating chamber is supplied by convection; that is, by the circulation of hot gases through the heat-treating chamber !!IA. A small amount of heat, however, is supplied by radiation and the manner in which the radiated heat assists in controlling the temperature will now be pointed out by reference to an operation of the furnace in which the temperature within the heat treating chamber was maintained in actual practice at a temperature of 922 F., plus or minus two degrees.

When the furnace has been set to maintain this temperature the control unit 55 will engage the L contact when the temperature of the thermocouple T! or TS is at 918 F. and will engage the H contact when the temperature of any one of these thermocouples is 922 F. Assuming now for the purpose of illustration that the furnace has been set into operation when cool, as previously described, and that the temperature within 'the heat-treating chamber !OA has now approached 922 F., plus or minus, referring particularly to Figs. 1 and 2, and assuming that the heating element E! is at this time energized, in addition to hot gas being circulated through the section of the urnace opposite the heater El, this section is also being heated by heat radiated through the curtain walls 25 adjacent the heating element El. From actual test it has been determined that the temperature on the inner surface of the curtain walls 25 at points adjacent the heating elements E! as indicated at points !30, ranges between 922 F. to 925 F. when the temperature at the thermocouple T! ranges from 9l8 F. to 922 F. When the section of the heat-treating chamber !!!A controlled by the heating elements E! has been raised so that the temperature at the thermocouple T! is 922 F., and when the parts of the control apparatus are in positions to connect the parts of the apparatus in operative relaticnship to the heating elements E! and the thermocouple T!, the apparatus wi l be energized so that the heating elements E! are de-energized. In other words, the parts are moved from the relative positions as illustrated in Fig. 5 into such positions that the various switches 16, 11, and 83 are opened and so that the contact 86 is in engage'ment with the contact 81. At this time the coi! 89 is energized and the valve Vi is moved from a closed position into an open position so that cooling gas is supplied through the pipes or ducts .12 associated with the section of the 'furnace controlled by the heating elemente Ei. As explained heretofore, this cooling gas is withdrawn from the lower forward part of the heat-treating chamber iiiA, which is probably the coolest point in the furnace, and it has been found from actual test that the cooling gas introduced through the pipes 43 ranges between a temperature of 913 F. to 917 F. The introduction of this cooling gas is efiective in cooling the hot gas which passes downwardly into the section of the iurnace underdiscussion and efiectively prevents the temperature from coasting upward. It appears 'that this cooling gas Compensates ;for the heat which has been stored up in the heating elements El and in the adjacent parts of the curtain walls 25. When the temperature in this section drops so that the temperature at the thermocouple Ti is 918 F., the apparatus is then operated so that all of the parts move into the positions illustrated in Fig. 5 and the heating elements El are energized, and the flow of cooling gas is either completely shut off or shut oil to a substantial extent according to the particular adjustment of the Valve Vi.

The temperature in any upper section of the iurnace, such as is controlled by the heating elements EH, is substantially the same as described with relation to the lower section controlled by the heating elements Et. It has been found in actual practice that when any upper heating element or pairs of heating elements are energized, the temperature immediately above it is approximately 950 F. while the temperature is reduced to 928 F. when the adjacent heating element is de-energized. In the operation of the heat treating furnace as just described it is possible to control the temperature within close limits, such for example as 922 F., plus or minus two degrees. The temperature in the upper part of the chamber is at times slightly hotter than the temperature in the lower part of the chamber, but during actual tests it has been found that this temperature does not vary more than 3, with the result that parts placed in any portion of the heat-treating chamber EDA are subjected to substantially the same temperature so far as practical results are concerned.

In Fig. 6 I have shown an alternative form of my invention designed for use insmaller furnaces. This form of my invention utilizes a curtain wall on but one side of the chamber o and in place of having the parts, such as the heating elements E, the pipes 42 'and 44, etc. in duplicate, they are provided only at one side of the furnace. There is, therefore, but one circula tint path of hot gas for heating the heattreating chamber MA and there is radiated beat applied from but one side of the heat-treating chamber MA. In Fig. 6 the parts are numbered to correspond to identical parts of the form of my invention illustrated in Figs. 1 to 5 inclusive, and the Construction and operation may be readily understood from the foregoing description when it is taken into consideration that the parts mentioned heretofore are not provided in duplicate.

My invention of electric furnace resides in the novel combination of elements defined heretofore and resides in a new princlple of operation per- :formed by such novel combination of parts. I therefore do not wish my invention to be limited to the details of Construction nor to the aoraseo particular combinatlon which comp'ises the preerrecl form of my invention. My invention likewise resides in the novel method described heretofore which may be practiced either in the two furnaces which I have illustrated or in other alternative constructions designed for practicing the method. For these reasons I desire my invention to be broadly construed in accordance with the spirit and scope of the appended claims.

I claim as my invention:

1. A heat treating furnace of the character described, including: walls forming a heat treating chamber and forming a gas heating chamber connected to said heat treating chamber at the upper and lower parts thereof; heating means in said gas heating chamber whereby the gas is heated, said heated gas flowing from said gas heating chamber into the upper end of said heat treating chamber, downwardiy therethrough and into said gas heating chamber; and gas circulating means for withdrawing gas from one part of said heat treating chamber and delivering it to other hotter parts thereof.

2. A heat treating iurnace of the character described, including: walls forming a heat treating chamber; hot-gas circulating means for circulating heated gas through said heat treating chamber for 'heating same; and cooling gas circulating means for withdrawing gas from one part of said heat treating chamber and delivering it to said heat treating chamber where the temperature has risen above a predetermined degree.

3. A beat treating furnace of the character described, including: walls iorming a chamber; a heating element disposed adjacent one wall of said chanber within said chamber; a curtain wall disposed between said heating element and the central space of said chamber, there being openings at the lower and upper edges oi' said curtain wall for the circulation ol' air upwardly along the side of said curtain wall facing said heating element and downwardly on the opposite side oi said curtain wall; and Conduit means i'or circulating air from the lower portion oi' said chamber to the upper portion thereof.

4. A heat treating Iurnace oi the character described, including: walls Iorming a chamber; heat producing means in said chamber disposed adjacent opposing walls of the chamber; curtain walls of beat obstructing material disposed relatively close to said last-named walls of said chamber and between said heat producing means and the central space o' said chamber, there being openings at the lower and upper edges oi' said curtain walls through which air may circulate upwardly across the faces of said curtain walla racing said beat produclng means and downwardly within the central space of said chamber; and means comprising a. pump and a piurality of conduit members for cirulating air from a. cooler part of said chamber to a. plurality of other hotter parts of said chamber.

5. A beat treating furnace ot the character described, including: walls Iorming a chamber; a heating element disposed adjacent one wall oi said chamber within said chamber; a curtain wall disposed between said heating element and the central space ot said chamber, there being openings at the lower and upper edges of said curtain wall for the circulation oi' air upwardly along the side oi' said curtain wall racing said heating element and downwardly on the opposite side of said curtain wall; and means comprising a pump and a plurality oi conduits extending outside said chamber tor circulating u: !rom the lower part of said chamber to a plurality of points in said chamber above the bottom thereoi.

6. A heat treating furnace of the character described, including: walls Iorning a heat treating chamber and forming a gas heating chamber connected to said heat treating chamber at the upper and lower parts thereof; heating means in said gas heating chamber whereby the gas is heated, said heated gas flowing from said gas heating chamber into the upper end of said heat treating chamber, downwardly therethrough and into said gas heating chamber; and gas delivery means for recirculating gas from a cooler part of said heat treating chamber and introducing it at locations where the temperature tends to increase above a desired degree.

7. A heat treating -iurnace of the character described, including: walls forming a heat treating chamber; a plurality of heating means whereby different sections of said heat treating chamber may be heated; control means for controlling the operation of saidheating means in accordance with the temperatures in the sections of said heat treating chamber; and means fr introducing cooling gas into a section of said heat'treating chamber when the heating means associated therewith has been made .non-operative by said control means.

' control means of said 8. In a method oi heat treating work, the steps of: placing the work to be heat treated in a heat treating chamber; circulating hot gas in a closed circuit through a gas heating means and through said heat treating chamber: and withdrawing gas !rom one part of said heat treating chamber and introducing it at another hotter part thereoi.

9. A heat' treating furnace of the character described, including: walls forming a chamber; a plurality of heat producers supplying heat to different parts of said chamber; a plurality of heat responsive elements in said chamber, one each being associated with each of said heat producers;

a plurality of air circulating conduits, each of which is placed so as to take air from the interier of said chamber and to deliver the same to a separate section of said chamber; a plurality of heat controlling members, one each of which is operatively connected to one of said heat producers for control of the same; a main control unit adapted for connection separately to each oneof said heat responsive elements and acting in accordance with the temperature of the same to adjust ,the heat producer associated therewith; means for connecting said main control unit individually to each of said heat responsive elements and its associated heat producer; flow control valves for said conduits; and means connected with said heat producers tor automatically opening and closing said valves.

10. A heat treating furnace of the character described, including: walls Iorming a chamber; a plurality oi heat producers disposed in adjacent sections of said chamber so as to deliver heat into said sections; a plurality of heat responsive elements in said chamber, one each being associated with each of said heat producers; a plurality of heat controlling members, one each of which is operatively connected to one of said heat producers for control of the same; a main control unit adapted for connection separately to each one of said heat responsive elements and acting in accordance with the temperature of the same to adjust the heat producer associated therewith; means Operating automatically to connect said main control unit individually to each of said heat responsive elements and its associated heat producer; a plurality of conduits each of which is so placed as to draw air from an end of said chamber and deliver the same to a section of said chamber; and flow producing and controlling means for said conduits connected with said control members of said heat producers, said lastnamed means being so connected to and controlled by said main control unit that the flow through said conduits will be maximum when the interior of said furnace is at maximum temperature.

11. A heat treating furnace of the character described, including: walls forming a chamber; a plurality of heating elements arranged in separate sections in .the interior of said chamber; control switches, each of which is connected to a separate section of`said heating elements and is adapted to connect the same to an electric current supply; a plurality of thermocouples, each of which is placed in said chamber in association with one of the sections of said heating elements; a main control unit adapted to be separately connected to each of said thermocouples and its associated control switch, said main control unit having electrical means acting' in response to the electric energy received from said thermocouples to actuate said control switches; means for connecting said main control means separately to each of said thermocouples and its associated control switch; air circulating means for said chamber comprising a plurality of conduits each connected so as to draw air from the lower part of one end of said chamber and to deliver the same to a separate section of said chamber; electrically controlled valve means for said conduits; and means connecting said vaive means with said main control unit and Operating to cause the opening and closing of said valve means in accordance with prescribed temperature conditions in said chamber 12. A heat treating furnace of the character described, including: walls forming a chamber; a plurality of heating elements arranged in separate sections in the interior of said chamber adjacent a wall thereof; a curtain wall between said heating elements and the heat treating space of the chamber; control switches, each of which is connected to a separate section of said heating elements and is adapted to connect the same to an electric current supply; a plurality of thermocouples, each of which is placed in said chamber in association with one of the sections of said heating elements; a main control unit adapted to be separately connected to each of said thermocouples and its associated control switch, said main control unit having electrical means acting in response to the electric energy received from said thermocouples to actuate said control switches; means for connecting said main control means separately to each of said thermocouples and its associated control switch; air circulating means for said chamber comprising a plurality of conduits each connected so as to draw air from the lower part of one end of said chamber and to deliver the same to a separate section of said chamber adjacent the face of said curtain wall presented toward said heat treating space of said chamber; electrically controlled valve means for said conduits; and means connecting said valve means with said main control unit and Operating to cause the opening and closing of said valve means in accordance with prescribed temperature conditions in said chamber.

13. Ina method of heat treating work, the steps of: placing the work to be heat treated in a heat treating chamber; circulating hot gas through said heat treating chamher; and withclrawing gas from a. cooler part of said heat treating chamher and ntroduclng it into said heat treating chamber where the temperature has risen ahove a predeterminecl degree.

14-. In a method of heat treating work, the steps of: placing the work to he heat t'eatecl in a heat treating chanher; circulating hot gas in two separate closecl circuits upwardly through a gas heating means from opposite sides of said heat treating chamber and downwardly through said heat treating chamher; and withdrawing gas from one part of said heat treating chamber and introducing it at another hotter part thereof 15. In a method of heat treating work, the steps of: placing the worh to he heat treated in a heat treatng chamber; circulating hot gas in two separate closed circults through a gas heating means from opposite sides of said heat treatng chamher and through said heat treating chamher; and withdrawing gas from a coole' part of said heat treatng chamher and introducing it at a hotter part thereof.

16. A heat treating turnace of the character described, including: walls t'orming a heat treating chamber and formng a gas heating chamher connected to said heat treating chamber at the upper and lower parts thereof; heating means in said gas heating chamher whereby the gas is heated, said heated gas fiowing from :said gas heating chamber into the upper end of said heat treating chamber downwardiy therethro'gh and into said gas heating chamber; gas circulating means i'orwithdrawing gas from one part of said heat treating chamber and delivering it to other parts thereof; and control means for controlling the operation of said gas circulating means whereby gas is delivered to said 'heat treating chanber where the temperature has risen above a predeterminecl degree.

17. A heat treating Iurnace oi' the character described. including: walls forming a heat treating chamber; hot gas circulating means for circulating heated gas through said heat treating chamber for heating same; cooling gas circulating means for withdrawing gas from one part of homes@ said heat treating chamber and delivering it to other parts thereof; and control means for controlling the operation of said gas crculatng means whereby gas is delivered to said heat treating chamber where the temperature has risen above a predetermined degree.

18. In a method of heat treating work, the steps of: placing the work to be heat treated in a heat 'creating chamher; circulatng hot gas through said heat treating chamber; and withclrawng gas from one part of said heat treating ohamher and ntroducing same at locations in said heat treating chamber where the temperature has risen above a predetermined degree.

19. In a method of heat treating work, the steps of: placing the work to be heat treated in a heat treating chamber; circulating heating gas through said heat treatng chamber; heating said heating gas by a plurality of heating means so that dfierent heating means heat said heating gas which is crculated through difierent sections of said heat treating chanber; de-energizng a heating means for a given section when said secton rises in temperature above a predetermned degree; and also introducing cooling gas into said last-named section when the heating means asociated therewith is de-energized.

20. In a heat treating furnace of the class described, the combinationof: walls forming a chamber; a curtain wall in said chamber, forming same into a heat treating chamber adapted to receive work, a heating chamber, and upper and lower passages connecting said heat treating and said heating chambers together; heating means in said heating chamber whereby gas is heated and caused to flow by convection in a closed circut through said heating chamber, said upper passage, said heat treating chamber, and said lower passage; and gas circulating means for withdrawing gas from one part of said heat treating chamber and introduced it into said heat treating chamber at one or more places at which the temperature has risen above a predetermined degree.

LEWIS NEWELL DAVIS. 

